Add unchecked transaction envelopes and EIP-4844 support - #78

Summary

• add public structurally decoded transaction envelopes for legacy and types 1 through 4
• keep parity, V, R, and S semantic validation in explicit checked conversions
• add EIP-4844 transaction/message support plus TransactionV4, BlockV4, and ReceiptV5
• add signer recovery for checked transactions, including high-S normalization for pre-fork-valid signatures
• reject trailing RLP data, malformed access-list structure, and contract creation for types 3 and 4
• bump the crate version to 0.19.0

Validation

• cargo test --all-features: 34 passed
• no-default, serde, and SCALE feature checks
• clippy with all features and targets under -D warnings
• rustdoc under -D warnings
• cargo publish --dry-run --allow-dirty

Follow-up

The companion rust-ethereum/evm PR consumes ethereum 0.19.0 for the new EELS Cancun runner, so this crate release needs to be published before that PR can pass without a local Cargo patch.